Mars wasn’t always the cold, dry, and dusty planet that NASA‘s rovers and landers are currently exploring. By mapping the mineral composition of wide swaths of the planet’s surface, researchers have discovered that parts of Mars once coursed with liquid water, which increases the possibility that it supported microbial life.

Using NASA’s Mars Reconnaissance Orbiter, researchers searched for traces of phyllosilicates, or clay-like minerals that preserve a record of water’s interaction with rocks. They found phyllosilicates in thousands of places, in valleys, dunes and craters in the ancient southern highlands, pointing to an active role by water in Mars’s earliest geological era, the Noachian period, 4.6 to 3.8 billion years ago [AFP].

The clay minerals were formed at low temperatures (100-200°C) – an important clue to understanding the Red Planet’s potential for habitability during the Noachian period. “What does this mean for habitability? It’s very strong,” [researcher John] Mustard said. “It wasn’t this hot, boiling cauldron. It was a benign, water-rich environment for a long period of time” [Telegraph]. Mustard’s team says that the clay formations are a promising place to look for ancient Martian microbes that might have inhabited the water, although other researchers have argued that Martian water was probably too salty to support life.

The findings, presented in the journal Nature [subscription required], provide the clearest picture yet of planet-wide hydrological impacts [Wired News]. The results add historical context to the work being done by the Mars Phoenix Lander, which is currently drilling into the frozen soil near the north pole to collect ice and dirt samples for analysis. But researchers say they still don’t understand the transition that turned Mars from a watery place to the mostly arid planet it is today, with water ice lurking only beneath the surface. The leading theory is that the planet’s once-thick atmosphere began to thin, causing the precious liquid to evaporate into space. Only a thin atmosphere, consisting overwhelmingly of carbon dioxide, remains today [AFP].